Free shipping on a quantity purchased would be a VERY nice thing to have.
Although there are some suggested workarounds, they simply don't work for most situations.
In the case of creating a separate product with the number of multiple items in it and designating it as free shipping: we have 25 different videos to choose from. If we wished to offer free shipping to someone ordering any 3 of them, there are simply too many possible combinations to make creating a separate product containing 3 items workable.
As for discounting the four items to roughly equal the free shipping, the problem is what if the customer then orders 5 or 6 items? That discount would extend to all items, effectively offering a discount beyond just free shipping.
Please, please, please, a reward for free shipping based on quantity of items.